Unique index on Build ref column

This is being added to prevent the race condition of multiple build requests for the same sha.

The index is global across all repositories so Kochiku now has the artificial restrictions that two different repositories can not have a build for the same sha. Is this going to be a problem? I could only imagining this happening if someone were to add the same repository to Kochiku twice since the chance of collision across multiple repositories is very small.

I also added pry-byebug to the Gemfile for debugging.

To: @cheister @square/kochiku-contributors